Hey Runz,
So basically, DE:HR and Metro 2033 are both Steamworks games, which means they require Steam in order to run.
If you have physical box copies of the games, then the simplest way to install them would be to open up Steam, log into your wife's account, and then insert the game disc(s) into your DVD drive.
If all goes according to plan, an auto-run dialog should pop up on your computer. You can start the installation from there, and it should install the game to your Steam directory in the Windows Program Files by default.
Sometime, during the course of installation, it will ask you for an activation key (or a serial number). Punch in the serial number located somewhere on a slip of paper inside your game box. This will permanently tie the game to your wife's Steam account. Once that happens, it cannot be activated again.
Alternatively, you can skip the disc-based install, and simply insert the activation key into Steam using the method KaOTiK described above ("Add a Game" > "Activate a Product on Steam".) This will allow you to simply download the game from Steam's servers, like any other Steam game.
The Steam client is a fairly non-intrusive form of DRM. It does, however, require internet access to activate your newly-purchased games; and in most cases, you'll have to log in in order to play.
Fortunately there is an Offline mode that allows you to play your game without signing in ("Steam" > "Offline".) It's been spotty for some people in the past, but it's mostly fixed now and it should work. Give it a try and let us know what happens.
I hope that helps.